I. Field of the Invention
This invention relates to a process for using dross residues to produce refractory products.
II. Description of the Prior Art
Dross is a material which forms on the surface of molten aluminum or aluminum alloys during remelting and metal holding and handling operations when the molten metal is in contact with a reactive atmosphere. Dross normally consists of metal oxides and nitrides and a considerable quantity of molten free (unreacted) metal. For economic reasons, it is normally desirable to extract the free metal for re-use, but afterwards the remaining dross (dross residue) has in the past been dumped in land fill sites since it has had no economic value.
The dumping of dross residues in this manner has recently come to be regarded as environmentally unacceptable because harmful chemicals may leach out of the dross into the water table and thus cause widespread pollution.
Part of the problem caused by dross residues results from the presence of soluble salts in the residues as a consequence of the traditional process for removing the free metal. This process involves heating and tumbling the dross in the presence of a molten salt bath in order to remelt the metallic fraction and to cause the resulting small molten metal droplets to coalesce and form an easily separable molten metal pool. While the process is quite efficient in extracting the metal, the residual salt cake forms a large proportion of the dross residue.
We have previously devised a process for extracting the free metal component from dross without the use of molten salt baths. This process is disclosed in our Canadian patent number 1,255,914 issued on Jun. 20, 1989 (the disclosure of which is incorporated herein by reference) and involves the treatment of dross in a furnace heated by means of a plasma torch. This heating procedure can be carried out on the dross without any prior treatments and results in the coalescence of molten metal droplets in the dross in the absence of molten salts.
While our improved process of treating dross avoids the environmental problems caused by the salt content of dross treated in the traditional way, the resulting "plasma dross" residue still contains appreciable amounts of aluminum nitride and may therefore still be unsuitable for disposal in land-fill sites since this chemical may also be a pollutant.
There is therefore a need for a process of disposing of dross which does not result in environmental problems and yet can be operated economically. Proposals have been made in the past to convert dross into useful products, such as refractories, with the intention of not only avoiding disposal problems but also of bringing in an economic return, but these proposals have not been put into widescale use because various problems have been encountered.
The use of dross residues to produce ceramics is discussed in an article entitled "Refractory Products Obtained Using Aluminum Rich Recovered Raw Materials" by R. Dal Maschio et. al. in Ceramics Developments, 34-36 (1988), pp. 735-739. The article investigates the formation of alumina-rich or spinel-based refractories by calcining dross either alone or following the addition of magnesium carbonate. In those cases where magnesium carbonate was employed, the dross was first calcined in air at about 800.degree. C., mixed with magnesite and then fired at temperatures of 300.degree.-600.degree. C. In those cases where the dross was used alone the material was first heated at 800.degree. or 1200.degree. C. before being fired. The preliminary heating or calcination step is necessary to convert the various compounds in the dross, particularly aluminum nitride, to the corresponding oxides and to smelt and evaporate some of the sodium chloride content. It was deduced from the tests that the dross needs a lengthy stay (some hours) at temperatures higher than 1200.degree. C. to reach a complete oxidation and a stable weight. A conclusion was reached that, in spite of a high impurity content, the calcined material could be used alone or mixed with magnesite for the production of refractory chammottes.
The problem with this conventional procedure is that it requires the preliminary calcination step and this is both time consuming and expensive in terms of energy. It is therefore believed that the process will not be put into widescale use.
Accordingly, there is still a need for a process for converting dross to useful products on a commercially viable scale and it is an object of the present invention to satisfy this need.